In the scene in the restaurant right before Chunk spills the water bottle, Mikey hits the floor with a metal bar. Brand (played by Josh Brolin) goes to him and says "..there is nothing buried under there..." and Mikey says, "There IS something buried under there, Josh..." using the actor's real name.
There is another instance of this same slip-up. Earlier in the movie when all of the Goonies are in the attic, Brand and Data are playing with the Electric Ball (I don't know what else to call it). Data says, "Don't touch it Josh" instead of addressing him as Brand. I don't remember the other scene in the basement but I will check it out.